The first estimates of the severity of the H7N9 influenza virus show that about one-third of people who were hospitalized with the infection died. And flu experts warn that the strain could reappear in the next flu season.

In February, Chinese health authorities first reported infections with the H7N9 influenza virus, a flu strain emerging from birds. While the virus did not seem to be as virulent as previous avian strains, public-health officials were concerned that it was the first time cases of H7N9 had been documented in humans.

According to the World Health Organization’s (WHO) H7N9 report in early June, there have been 132 lab-confirmed cases of human H7N9 infection in China. The majority have been reported in middle-aged men, most of whom had some exposure to poultry, and by June, 37 people had died from the disease.

Flu scientists say that so far there is little evidence that the virus easily spreads from person to person. But they are not ruling out the possibility, since a few cases appeared to result from an infected person passing on the infection during close contact, such as occurs among family members or health care workers. Based on previous studies that confirm how easily influenza viruses can mutate, researchers are also concerned that H7N9 could morph to become more transmissible.

“Should sustained human-to-human transmission occur with an increased number of clinically severe cases, health systems are likely to be strained. WHO is providing coordination and guidance regarding provisional vaccine candidates; there are currently no recommendations on the large-scale manufacture of H7N9 vaccine,” the WHO reports.

Having such a plan may be critical during the coming flu season. In an analysis published in the Lancet journal, researchers from the Chinese Center for Disease Control and Prevention in Beijing and from the University of Hong Kong reported that H7N9 infection was fatal in about a third of cases that are severe enough to require hospitalization. The H5N1 avian virus that caused a worldwide pandemic in 2003 had a 60% fatality risk for those hospitalized, while the more recent H1N1 flu was associated with a 21% risk of death for those seeking hospital care.

The scientists based their H7N9 estimate on the number of people treated at hospitals throughout China, as well as information from their surveillance networks that monitor the volume of cases. Overall, they say that up to 3% of all people who became infected were at risk of dying from H7N9.

While H7N9 is less deadly than H5N1, the notorious genetic instability of influenza means H7N9 could re-emerge in the winter — possibly in a more virulent and deadly form.

“While H7N9 remains predominantly spread from poultry to humans with no or limited human-to-human transmission, it would be important to stay vigilant to such a possibility of human-to-human spread, if indeed the virus returns in the fall-winter, especially if live-poultry markets that have remained closed since the height of the outbreak were to reopen,” says study author Gabriel Leung, a professor in the department of community medicine at the University of Hong Kong.

The researchers’ concern about H7N9’s reappearance in the upcoming flu season is based on an in-depth comparison of H7N9’s disease pattern with that of H5N1 in the early 2000s. In another article they published in the same issue of the journal, they reported that H7N9’s incubation period is about three days, which is lower than previous estimates and critical for developing public-health measures to protect people from infection during the coming winter. The researchers also warn that the hospital cases represent only a small fraction of the total number of people affected by a particular influenza strain.

“The mo[st] relevant finding, regarding implications for the coming fall-winter, is that the clinical iceberg phenomenon that is common … of most flu viruses applies to H7N9 as well. We estimated that between 1,500 and 27,000 symptomatic infections might have occurred in China, compared to the 132 laboratory-confirmed cases,” says Leung.

But even with new cases of H7N9 infection waning, the authors write that the off-season for the disease gives health care workers an advantage:

The warm season has now begun in China, and only one new laboratory-confirmed case of H7N9 in human beings has been identified since May 8, 2013. If H7N9 follows a similar pattern to H5N1, the epidemic could reappear in the autumn. This potential lull should be an opportunity for discussion of definitive preventive public health measures, optimisation [sic] of clinical management, and capacity building in the region in view of the possibility that H7N9 could spread beyond China’s borders.

They note that the decline in infections could be largely due to the closing of poultry markets where the virus was believed to originate, but as these markets reopen, and colder weather rolls in, the threat of H7N9 could resurface. The Chinese health bureau is monitoring suspicious flu cases, and officials at schools and hospitals are also on alert for flulike symptoms and respiratory illnesses, since influenza viruses tend to spread more quickly among students and health care workers. So far, WHO continues to advise countries with cases of the virus to remain vigilant, but has not issued travel warnings or restrictions to these regions.

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